Fluorescence and phosphorescence properties of the low temperature forms of the MAl2Si2O8:Eu2+ (M=Ca, Sr, Ba) compounds

Crystal phase transition and polyhedron transformation towards the evolution of photoluminescence and the improvement of thermal stability in efficient blue-emitting Ba0.47-xSr0.50+xAl2Si2O8:Eu2

Investigations into novel single-phase phosphors with outstanding luminescence properties and excellent thermal stability are urgently needed in the lighting field. In this work, a crystal phase transition and polyhedron transformation strategy via cation substitution has been proposed. Via controlling the Sr/Ba ratio, the structural evolution of the phosphor from a monocelsian phase to a hexacelsian or feldspar phase and the variation of the local environments of Eu²⁺ sites are correspondingly studied in Ba_0.47-xSr_0.50+xAl₂Si₂O₈:0.03Eu. Consequently, the optimal Ba_0.17Sr_0.80Al₂Si₂O₈:0.03Eu sample exhibits a higher intensity, up to 15.2-fold that of Ba_0.97Al₂Si₂O₈:0.03Eu. A narrower full-width-at-half-maximum of 73 nm, better color purity of 82.96%, and an internal quantum yield of 82.3% can be realized. With an increase in temperature, the emission intensity losses of samples from x = -10.0-47.0% are no more than 10.0% at 473 K. Moreover, a WLED (CCT = 5210 K; CRI = 90.3) fabricated using Ba_0.17Sr_0.80Al₂Si₂O₈:0.03Eu displays warmer white light than one fabricated using BaMgAl₁₀O₁₇:Eu under the same assembly and test conditions. Analysis shows that the structural evolution with reduced polyhedral symmetry and the condensed crystal structure with fortified rigidity are responsible for the improvement in properties. This discovery demonstrates that the utilization of a crystal phase transition and symmetrical coordination is an efficient way to develop novel efficient phosphors and other related materials.

Hexagonal Sr1-x/2Al2-xSixO4:Eu2+,Dy3+ transparent ceramics with tuneable persistent luminescence properties

Co-doped hexagonal Sr1-x/2Al2-xSixO4:Eu2+,Dy3+ (0.1 ≤ x ≤ 0.5) transparent ceramics have been elaborated by full glass crystallization. The compositions with low SiO2 content (x ≤ 0.4) require fast quenching conditions to form glass, i.e. specific elaboration processes such as aerodynamic levitation coupled to laser heating, whereas the x = 0.5 glass composition can be prepared on a large scale by the classic melt-quenching method in commercial furnaces. After a single thermal treatment, the resulting SrAl2O4-based transparent ceramics show varying photoluminescence emission properties when x increases. These variations are also observable in persistent luminescence, resulting in an afterglow colour-tuning ranging from green to light blue. Afterglow excitation spectra highlight the possible activation in the visible range of the obtained persistent luminescence. Indeed, persistent luminescence of hexagonal Sr0.75Al1.5Si0.5O4:Eu2+,Dy3+ large transparent ceramics has been successfully charged using a typical smartphone low power white light source. Moreover, thermoluminescence glow curves of samples containing different Dy3+ doping concentrations are studied to gain insights regarding the traps' origin and depth. Coupling thermoluminescence results together with luminescence thermal quenching and band gap calculations appear useful to understand the charge trapping and detrapping evolution with the material composition. Varying the Si-content in hexagonal Sr1-x/2Al2-xSixO4:Eu2+,Dy3+ compounds appears as a promising strategy to obtain transparent materials with tuneable green to light blue persistent luminescence.

Yellow Persistent Phosphor Ba13.35Al30.7Si5.3O70:Eu2+,Tm3+ from the Energy Regulation of Rare-Earth Ions

The luminescence properties of Ba13.35Al30.7Si5.3O70:Eu2+ and Ba13.35Al30.7Si5.3O70:Eu2+,Tm3+ phosphors are presented. After being excited by a light source, Ba13.35Al30.7Si5.3O70:Eu2+,Tm3+ phosphors emit intense yellow long persistent luminescence covering the region from 450 to 700 nm, which can last about 8 h. Thermoluminescence curves were demonstrated to analyze the trapping nature of persistent luminescence. Tm3+ is added to improve the long persistent luminescence properties of phosphors. The mechanism of persistent luminescence has been studied.

Characterization of Luminescent Materials with 151Eu Mössbauer Spectroscopy

The application of Mössbauer spectroscopy to luminescent materials is described. Many solids doped with europium are luminescent, i.e., when irradiated with light they emit light of a longer wavelength. These materials therefore have practical applications in tuning the light output of devices like light emitting diodes. The optical properties are very different for the two possible valence states Eu 2 + and Eu 3 + , the former producing ultraviolet/visible light that shifts from violet to red depending on the host and the latter red light, so it is important to have a knowledge of their behavior in a sample environment. Photoluminescence spectra cannot give a quantitative analysis of Eu 2 + and Eu 3 + ions. Mössbauer spectroscopy, however, is more powerful and gives a separate spectrum for each oxidation state enabling the relative amount present to be estimated. The oxidation state can be identified from its isomer shift which is between - 12 and - 15 mm/s for Eu 2 + compared to around 0 mm/s for Eu 3 + . Furthermore, within each oxidation state, there are changes depending on the ligands attached to the europium: the shift is more positive for increased covalency of the bonding ligand X, or Eu concentration, and decreases for increasing Eu⁻X bond length.

Effects of changing the M2+ cation on the crystal structure and optical properties of divalent samarium-doped MAl2Si2O8 (M = Ca, Sr, Ba)

The photoluminescence properties and theoretical analysis of Sm²⁺ doped MAl₂Si₂O₈ (M = Ca, Sr, Ba) were reported. The theoretical analysis is in good agreement with our experimental results.

Structure and photoluminescence characteristics of europium(iii) doped in CaAl2Si2O8phosphors

A series of Eu³⁺activated CaAl₂Si₂O₈phosphors have been synthesized at 1350 °C in air, and their photoluminescence properties have been investigated as a function of activator concentrations.

Luminescent Afterglow Behavior in the M₂Si₅N₈: Eu Family (M = Ca, Sr, Ba)

Persistent luminescent materials are able to emit light for hours after being excited. The majority of persistent phosphors emit in the blue or green region of the visible spectrum. Orange- or red-emitting phosphors, strongly desired for emergency signage and medical imaging, are scarce. We prepared the nitrido-silicates Ca₂Si₅N₈:Eu (orange), Sr₂Si₅N₈:Eu (reddish), Ba₂Si₅N₈:Eu (yellowish orange), and their rare-earth codoped variants (R = Nd, Dy, Sm, Tm) through a solid state reaction, and investigated their luminescence and afterglow properties. In this paper, we describe how the persistent luminescence is affected by the type of codopant and the choice and ratio of the starting products. All the materials exhibit some form of persistent luminescence, but for Sr₂Si₅N₈:Eu,R this is very weak. In Ba₂Si₅N₈:Eu the afterglow remains visible for about 400 s, and Ca₂Si₅N₈:Eu,Tm shows the brightest and longest afterglow, lasting about 2,500 s. For optimal persistent luminescence, the dopant and codopant should be added in their fluoride form, in concentrations below 1 mol%. A Ca₃N₂ deficiency of about 5% triples the afterglow intensity. Our results show that Ba₂Si₅N₈:Eu(,R) and Ca₂Si₅N₈:Eu(,R) are promising persistent phosphors for applications requiring orange or red light.

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20-40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.

Persistent Luminescence in Eu2+-Doped Compounds: A Review

In 1996, Matsuzawa et al. reported on the extremely long-lasting afterglow of SrAl₂O₄:Eu²⁺ codoped with Dy³⁺ ions, which was more than 10-times brighter than the previously widely used ZnS:Cu,Co. Since then, research for stable and efficient persistent phosphors has continuously gained popularity. However, even today - almost 15 years after the discovery of SrAl₂O₄:Eu²⁺, Dy³⁺ - the number of persistent luminescent materials is still relatively low. Furthermore, the mechanism behind this phenomenon is still unclear. Although most authors agree on the general features, such as the existence of long-lived trap levels, many details are still shrouded in mystery. In this review, we present an overview of the important classes of known persistent luminescent materials based on Eu²⁺-emission and how they were prepared, and we take a closer look at the models and mechanisms that have been suggested to explain bright afterglow in various compounds.